The molecular mechanisms involved in the metabolic activation of certain cytotoxic furans, namely 4-ipomeanol, a natural product isolated from mouldy sweet potatoes, and 3-methylfuran (3-MF), an atmospheric pollutant, are being investigated. Not only does the environmental occurrence of certain furans possibly have major toxicological significance for mankind, there is also a growing interest in some furan derivatives as potential antitumor agents. Oxygen and NADPH dependent metabolic activation of these furans results in the formation of highly electrophilic metabolites which alkylate microsomal proteins. Acetylacrolein (AA), a product of peracid oxidation of 2-methylfuran (2-MF), a naturally occurring cytotoxic furan, was investigated as a possible microsomal metabolite. Glutathione, N-acetylcysteine and cysteine adducts of AA although apparently rapidly formed, were not stable enough to allow isolation. When rat liver microsomal incubations were performed with semicarbazide (SC), NADPH and 2-MF, a metabolite was isolated by gel permeation chromatography and HPLC and was found to be identical by mass spectroscopy to the disemicarbazone of AA. Methylbutenedial, the analogous oxidative ring opened product from 3-MF was isolated as the disemicarbazone from rat liver microsomal incubation with 3-MF. Addition of SC to rat lung microsomes containing 3H-3-MF, and NADPH significantly inhibited covalent binding of 3H-3-MF metabolites to microsomal proteins, although a considerably greater amount of 3-MF was metabolized in the presence of SC. Coordinate with the decreased covalent binding there was the appearance of the radiolabeled semicarbazide conjugate of methylbutendial. Thus, the dialdehyde appears to be the reactive metabolite of 3-MF that is capable of binding covalently to tissue macromolecules. Moreover, since the covalent binding of reactive material is directly correlated with the occurrence of toxic lesions, the dialdehyde is probably the ultimate toxic metabolite involved. Future studies will continue to explore this hypothesis and will be extended to other cytotoxic furans including 4-ipomeanol and perilla ketone.